But Shorr says that was a general election; in a Republican primary, Romney will have a much harder job.

"Here's Romney's challenge ... in a lot of these primaries you're dealing with an electorate that's made for Santorum," Shorr says. "They have questions about Romney's conservatism, and they don't have any questions about Santorum's.

"Look, when Romney's folks took out Newt Gingrich ... it was really like shooting fish in the barrel."

Unlike Gingrich, Shorr says, Santorum isn't a target-rich environment. But Charlie Black, an outside adviser to the Romney campaign, isn't so sure. He says people are overestimating Santorum's surge.

"The polls you see right now reflected the bounce, the halo effect he got from winning three nonbinding events last week," Black says. "Santorum really hasn't had a glove laid on him yet, but his record and his position on the issues will be examined now."

The Romney team is just getting started with millions of dollars in ads in many of the upcoming primary states, including Romney's native state of Michigan, where a loss to Santorum would be humiliating.

The Santorum campaign is trying to inoculate itself against the assault with its own ad that shows an image of Santorum dodging a mudslinging rifle attack.

"We're not going to sit back and take incoming from the Mitt Romney campaign that's dishonest and untruthful and negative. That's not what the country wants to hear," says Hogan Gidley, Santorum's national communications director. "I think they've roundly rejected that to this point, as you can tell by Mitt Romney's troubling favorable ratings while Rick Santorum sits in the 70 percentile."

In Michigan, for instance, Santorum's favorable ratings are much higher than Romney's — 67 percent to 49 percent, according to Public Policy Polling. And that's why Richard Land, the head of the Southern Baptist Convention's Ethics and Religious Liberty Commission, says Romney needs to be careful.

"If he starts trying to attack Rick personally, the way he attacked Newt [Gingrich], it's going to be like, you know, big mean Mitt jumping all over the choir boy," Land says. "There could be a real backlash."

Reaching Conservative Voters

There's something else fueling Santorum's rise.

Social issues have returned to the debate thanks to President Obama's decision to require contraceptive coverage at religiously affiliated institutions. Land says that gives Santorum an edge.

"People know about Romneycare and they know that it was similar to Obamacare, and when the president takes off his mask and we see the ugly business end of government coercion, as we did last week, it reminds social conservatives of what's at stake," Land says.

In a Republican primary, Romney can't run to Santorum's left by arguing that Santorum is too conservative to beat Obama — even if that may be true. And, unlike Gingrich or even Rick Perry, Santorum doesn't offer Romney much room on his right.

Going Positive

Shorr thinks the Romney campaign is in a tough spot.

"I'm sure that they're poring over their polling, really trying to look for where to go against Santorum. Yes, he can try to paint Santorum as an insider in Washington, but he's really an insider of the corporate suite," Shorr says. "I just think that's a tough sell, and in some way shape or form, it may be that Romney's positive ads are going to matter as much as anything else."

Now that's an odd notion — that, this year, positive ads could make the difference. But plenty of Republicans say Romney has to do more to lay out a positive vision for the future. One way or another, Romney adviser Black is still counting on the candidate's superior resources.

"You're going into a period where the race becomes more national. There are actually 13 events including in some large states in the next three weeks," Black says. "One reason that Romney has withstood the challenge of four or five or six candidates already is he has a national political organization and a national fundraising organization. Nobody else does."

Black predicts that Romney's organization and money will prevail in the upcoming states. It's certainly true that when Romney has brought this advantage to bear, he has won — see Florida. When he hasn't spent big, he has lost — see Colorado, Missouri and Minnesota.

But it might take more than financial advantage for Romney to regain his front-runner status.

Copyright 2012 National Public Radio. To see more, visit http://www.npr.org/.